Increasing incidence of HIV and STIs among men who have sex with men (MSM) and transgender women (TGW) highlight the need for novel approaches to reducing infections-especially among individuals who do not use condoms consistently. Our prior studies on the use of rapid HIV self-testing (ST) by MSM who regularly engage in high-risk sexual behavior, show that these men are willing to use ST with partners, avoid intercourse in situations where partners test HIV-positive, and, over time tend to reduce their high-risk sexual behavior. This proposed study expands this line of research by exploring the use of a new device, the mChip dongle. Developed at the Columbia University School of Engineering, this device uses smartphone technology to analyze a drop of blood and test it for HIV and syphilis antibodies. The mChip dongle brings more precision to ST, tests for HIV and syphilis and potentially other STIs, and can capitalize on smartphone technology to provide clearer user instructions, on-hand information about HIV/STI, remote monitoring of the its use, and linkage of newly diagnosed individuals to care. For this proposed quantitative and qualitative study, we will again focus on HIV-uninfected MSM and TGW with multiple sexual partners who know how to prevent HIV infection, yet engage in high-risk sexual behavior. The Specific Aims are to: 1) Assess acceptability among MSM and TGW of using the mChip dongle to test sexual partners; 2) Optimize the smartphone app and mChip dongle for home use; and 3) Assess the feasibility of using the mChip dongle to test sexual partners, including: a) frequency of use of the mChip dongle with sexual partners over 3 months; b) factors associated with mChip dongle use or non-use; c) condom use with partners who were not screened vs. partners screened HIV or STI- negative with the mChip dongle; and d) factors associated with changes in sexual risk behavior over 3 months. The study consists of two phases. In Phase 1A, 60 participants will undergo a quantitative, qualitative, and skill-demonstration assessment (using the mChip dongle in front of the interviewer) in a single study visit. Phase 1A findings wil inform Phase 1B, which focuses on revising the smartphone app and mChip dongle for home users, as well as revisions to the Phase 2 assessment. In Phase 2A, 80 new participants will undergo a baseline quantitative and skill-demonstration assessment in order to screen for 50 participants who will be invited into the Phase 2B 3-month study. These 50 participants will be provided with an mChip dongle and given the choice to use it to screen sexual partners. After 3 months participants will undergo a quantitative and qualitative interview about experiences using (or not using) the mChip dongle with partners. This study will capitalize on a critical juncture in the development of the mChip dongle by systematically engaging its intended users in offering feedback to the engineering team on how to optimize its usability and acceptability among high-risk populations. This study will also allow us to assess the feasibility of conducting a larger and longer trial to develop and test an mChip-based harm-reduction intervention.